1. Field of the Invention
The present invention relates to a wavelength selective optical device used in a wavelength multiplexing method for superposing optical signals with a plurality of wavelengths and transmitting the optical signals through one optical fiber in an optical communication field. In particular, the present invention relates to a wavelength selective optical device employing an optical filter as a wavelength selecting element and a method of tuning the same.
2. Related Art
In optical communication field or the like, there are known various devices utilizing the nature of the wavelength of light for controlling transmission and transferring of information. For example, the wavelength division multiplexing large-capacity optical communication (Dense Wavelength Division Multiplexing (DWDM), Coarse Wavelength Division Multiplexing (CWDM), or the like) in which multi-wavelength laser beams with narrow line width are superposed at a high density and then input/output into/from one optical fiber is now spreading. In this wavelength division multiplexing optical communication, a desired wavelength signal (channel) must be demultiplexed from the multiplexed light signals or conversely multiplexed to such multiplexed light signals to bundle them into one optical fiber.
The center wavelength and the wavelength width of each channel is normalized respectively. The optical coupler constituting the system must select only a desired channel signal and pick it up at low loss, and must prevent crosstalks of unselected signals between its adjacent channels and its outside channels.
In a high-density wavelength division multiplexing system such as the DWDM, CWDM or the like utilizing the dielectric multi-layered optical interference filter as a wavelength division element, in order to enhance the isolation between the picked-ups signal and reduce the crosstalk between them, the light transmitted through the filter is used to select the signal and also the reflected light including the residual reflection is treated as the express signal for the optical coupler on the subsequent stage.
As an optical filter, various optical filters such as a band pass filter (BPF) for passing only a predetermined wavelength band, a shortwave pass filter (SWPF) for passing only a shorter wavelength side than the predetermined wavelength, a longwave pass filter (LWPF) for passing a wavelength longer than the predetermined wavelength, etc. are present in compliance with the applications. Normally, BPF is used in DWDM or CWDM.
Here, of importance are (1) both-side wavelength edges of the pass wavelength band of BPF are positioned on the outsides of the wavelengths on both ends of the selected channel, and a signal loss of the signal in all wavelength bands in the channel is small (2) both-side wavelength edges are positioned such that the pass wavelength band of BPF does not contain wavelength bands of adjacent unselected channels, and crosstalks of unselected channels are suppressed sufficiently small.
Characteristics of BPF such as the wavelength bandwidth, the crosstalk blocking amount (isolation), etc. are decided substantially by the filter design. However, values of the filter such as the edge wavelength, the center wavelength, etc. are varied every lot at the time of filter production. Also, these values are varied to some extent in a sheet of glass substrate. For this reason, an amount of wavelength shift must be tuned (the wavelength tuning must be applied) consciously at the time of assembling the optical coupler to make the optical coupler conform to the standard of the optical system (component) using this optical coupler as the constituent element.
For example, in the case of 100 GHz DWDM system, a channel interval is about 0.8 nm and a channel bandwidth is about 0.22 nm. Therefore, it is possible that the performance characteristic of the component is largely affected even by a small wavelength deviation such as about 0.1 nm.
FIG. 1 shows a concept to tune the center wavelength of BPF. Assume that, when an incident light is incident on the BPF manufactured based on predetermined design values in compliance with a predetermined method, the passing characteristic of BPF, illustrated by light intensity P, is given as indicated by a broken line. This indicates that the center wavelength is deviated from a center wavelength λp of the specified channel (x) to the longer wavelength side and that a loss is increased on the shorter wavelength side than the channel bandwidth of the channel (x) to be selected. Also, the crosstalk in the unselected channel (x+1) on the longer wavelength side is increased. Therefore, the wavelength tuning must be applied to this characteristic as indicated by a solid line by any method.
Meanwhile, in the dielectric multi-layered optical filter having the wavelength selectivity, when an incident angle of an incident light is changed, the wavelength edge is changed, or the center wavelength of the pass band together with the wavelength edge, if the filter is BPF, is changed. Normally the center wavelength is shifted to the shorter wavelength side by the oblique incidence in contrast to the vertical incidence. Consequently, it is feasible to execute the above wavelength tuning by utilizing this phenomenon.
FIG. 2 is a sectional view showing a basic structure of the wavelength division multiplexing optical coupler using a graded index rod lens. This optical coupler is assembled by optically tuning/fixing a subassembly, which is constructed by pasting an optical filter chip 40 onto a lens surface 33 of a dual optical fiber collimator 20, and a single fiber collimator 10. This dual optical fiber collimator 20 consists of a dual optical fiber pigtail 21 and a graded index rod lens 31. This single optical fiber collimator 10 consists of a single optical fiber pigtail 22 (dual optical fiber pigtail may also be employed) and a graded index rod lens 32.
An emitted light from one optical fiber 23 is incident on one end surface of the graded index rod lens 31. Assume that a lens length of the rod lens is a 0.25 pitch (¼ of a ray sinusoidal wave path period (pitch) peculiar to the graded index rod lens), an emitted light from the rod lens 31 is collimated into a parallel light beam. Then, a light contained in this parallel light beam in a predetermined wavelength range is reflected by the optical filter 40, then is converged again by the rod lens 31, and then is coupled to another optical fiber 24.
Also, a light other than lights in a light reflecting wavelength range is transmitted through the optical filter 40, then is converged by the rod lens 32 of the single fiber collimator 10, and then is coupled to the optical fiber 25. The signal light is wavelength-separated via such optical paths to reflect the optical characteristics of the filter.
As the prior art associated with the wavelength tuning of the filter, in U.S. Pat. No. 5,799,121, for example, the technology of changing the incident angle of the light into the optical filter by changing an alignment distance of two optical fibers to tune the center wavelength is set forth. In other words, an incident position of the light into the rod lens 31 is changed by changing a distance (an offset amount of the optical fiber, see FIG. 2) d between an optical axis of the pigtail 21 and optical axes of the optical fibers 23, 24, and thus the incident angle (φ) into the optical filter 40 is changed. When an optical fiber distance 2d (normally two optical fibers 23, 24 are arranged at an equal distance from the optical axis of the rod lens 31) is increased, the center wavelength of the selected signal is shifted to the shorter wavelength side.
Similarly, in U.S. Pat. No. 6,084,994, a mode of so-called dual optical fiber pigtail is constructed by fixing two optical fibers in the holder at a predetermined distance to actually suit the production of the optical coupler. Since the incident angle of the light into the optical filter can be changed by changing the distance between the optical axes of the optical fibers by exchanging the holder, it is possible to tune the center wavelength.
In the above method of tuning the selected center wavelength by adjusting the core distance of two optical fibers, there existed problems described in the following.
A core distance of two optical fibers becomes minimum when two optical fibers are tightly contacted to each other in parallel. A lower limit value of the core distance is defined by the cladding diameter (normally 125 μm) of the optical fiber. Since a finite effective aperture (a diameter which functions as the lens) exists in the rod lens, an upper limit value of the core distance is restricted by this effective aperture. Therefore, it is impossible to tune the selected center wavelength over the sufficient range.
Also, it is normal that the above holder is employed as the practical optical fiber fixing method. Normally the capillary in which through holes, through which the optical fiber is inserted respectively, are opened along the axis of the cylindrical member is employed as this holder. However, since an interval between the through holes is small particularly near the above lower limit value of the core distance, it is difficult to open two through holes while maintaining the core distance at a desired value.